Field of the Invention
This invention relates to apparatus for detecting the presence of a pager signal in a frequency modulated (FM) radio transmission.
The FM radio frequency band typically extends from about 87 to 104 MHz. Within this band there are a plurality of channels, each carrying a stereo multipleted signal or mono signal, to which is sometimes added a pager signal transmitted on a sub-carrier at a predetermined frequency. Typically, this predetermined frequency is 57 KHz, being phase locked to the third harmonic of the 19 pilot tone of the stereophonic multiplex signal.
When a receiver is scanning the channels for a paging signal, it detects the audio signal (music, speech etc.) and then checks for the existence of a pager signal at 57 KHz. The receiver thus scans through every channel transmitting an audio signal and checks for the existence of a page signal. This takes a substantial time because a pager signal is not transmitted on all channels. The invention aints to provide apparatus which speeds up the location of pager signals in a frequency modulated radio transmission.